The present invention relates to a lithographic printing plate material to which silver complex diffusion transfer process is applied for making printing plates.
A lithographic printing plate comprises an oleophilic image area which is receptive to oily inks and an ink-repellent non-image area which is not receptive to inks. In general, said non-image area comprises a hydrophilic area receptive to water. In usual lithographic printing, both water and ink are fed to the surface of the printing plate to allow the image area to receive preferentially the colored ink and the non-image area to receive preferentially water and then the ink on the image is transferred onto a material to be printed, such as paper.
Accordingly, in order to obtain prints of good quality, it is important that the difference of oleophilicity or hydrophilicity between the surface of the image and that of the non-image is sufficiently large, so that when water and ink are applied to the surface of the printing plate the image can receive sufficient amount of ink while the non-image completely repels the ink.
Lithographic printing plates made by silver complex diffusion transfer process (DTR process), especially, those which have a physical development nuclei layer on a silver halide emulsion layer are disclosed, for example, in U.S. Pat. Nos. 3,728,114, 4,134,769, 4,160,670, 4,336,321, 4,501,811, 4,510,228 and 4,621,041. The exposed silver halide crystal undergoes chemical development by DTR development to become black silver which serves as a hydrophilic non-image area. On the other hand, unexposed silver halide crystal is converted to a silver complex by a silver salt complexing agent contained in the developer and the complex diffuses to the surface physical development nuclei layer and physical development occurs due to the presence of nuclei to form an image mainly composed of ink-receptive physically developed silver.
In this way, in the lithographic printing plate, a silver image precipitated on the physical development nuclei layer provided on a gelatin-silver halide emulsion layer is utilized as an ink-receptive image and the lithographic printing plate suffers from the problems that its resistance against mechanical abrasion is insufficient as compared with general lithographic printing plates such as PS plate and the image is torn off or the ink-receptivity of the image is gradually lost.
If hardness of gelatin is enhanced or amount of the physical development nuclei is increased in order to overcome these problems, the background areas are stained resulting in serious decrease of printing endurance. Furthermore, staining of the background occurs or ink-receptivity deteriorates during storage of long period of from production of lithographic printing plate materials to making of printing plates therefrom and solution of these problems have been desired. As an approach to improve water retentivity (stain resistance), Japanese Patent Application No. Kokai No. 4-277748 proposes to contain at least one water-soluble synthetic polymer such as acrylamide polymer and/or acrylamide-acrylic acid copolymer in the physical development nuclei layer and/or a layer contiguous thereto. However, the improvement is still insufficient.